Characteristics and hazards of the cinnamaldehyde oxidation process

Article abstract of DOI:10.1039/c9ra10820c

Pressure and temperature behavior of the cinnamaldehyde oxidation process was determined using a custom-designed mini closed pressure vessel test (MCPVT), which is a new method to investigate the stability and hazard assesment of the cinnamaldehyde oxidation reaction. The oxidation products were analyzed by gas chromatography-mass spectrometry (GC-MS). The results showed that cinnamaldehyde was stable under nitrogen atmosphere but very unstable under oxygen atmosphere. The initial oxidation products were analyzed by iodimetry and the cinnamaldehyde peroxide value could reach 139.44 mmol kg^?1when the oxidation temperature was 308 K. The oxidation kinetics of cinnamaldehyde were studied by using the pressureversustime (P-t) curves obtained from the MCPVT process. The reaction is a second-order reaction, the kinetic equation is ln k= ?2233.66 × (1/T) + 11.19, and the activation energyE_ais 18.57 kJ mol^?1at 308-338 K. The explosion of the cinnamaldehyde oxidation reaction was observed by MCPVT, in which the onset temperature was 373 K. The main products of cinnamaldehyde oxidation are acetaldehyde, benzaldehyde, phenylacetaldehyde, acetophenone, 2-hydroxyphenyl acetone, cinnamaldehyde epoxide, benzoic acid, and cinnamic acid. Oxidation is a three-step process: (1) cinnamaldehyde reacts with oxygen to form peroxides; (2) complex oxidation reactions are caused by the thermal decomposition of peroxides; (3) rapid oxidation and thermal decomposition lead to explosion hazard.

Full text of DOI:10.1039/c9ra10820c

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Characteristics and hazards of the cinnamaldehyde
oxidation process
Cite this: RSC Adv., 2020, 10, 19124
Chang Yu,
Yuan-Lin Li,
Min Liang, Su-Yi Dai, Li Ma, Wei-Guang Li, Fang Lai
and Xiong-Min Liu*
Pressure and temperature behavior of the cinnamaldehyde oxidation process was determined using
a custom-designed mini closed pressure vessel test (MCPVT), which is a new method to investigate the
stability and hazard assesment of the cinnamaldehyde oxidation reaction. The oxidation products were
analyzed by gas chromatography-mass spectrometry (GC-MS). The results showed that cinnamaldehyde
was stable under nitrogen atmosphere but very unstable under oxygen atmosphere. The initial oxidation
products were analyzed by iodimetry and the cinnamaldehyde peroxide value could reach 139.44 mmol
ꢀ
1
kg when the oxidation temperature was 308 K. The oxidation kinetics of cinnamaldehyde were studied
by using the pressure versus time (P–t) curves obtained from the MCPVT process. The reaction is
a second-order reaction, the kinetic equation is ln k ¼ ꢀ2233.66 ꢁ (1/T) + 11.19, and the activation
ꢀ
1
energy E is 18.57 kJ mol at 308–338 K. The explosion of the cinnamaldehyde oxidation reaction was
a
observed by MCPVT, in which the onset temperature was 373 K. The main products of cinnamaldehyde
oxidation are acetaldehyde, benzaldehyde, phenylacetaldehyde, acetophenone, 2-hydroxyphenyl
acetone, cinnamaldehyde epoxide, benzoic acid, and cinnamic acid. Oxidation is a three-step process:
Received 23rd December 2019
Accepted 29th April 2020
(
1) cinnamaldehyde reacts with oxygen to form peroxides; (2) complex oxidation reactions are caused by
DOI: 10.1039/c9ra10820c
the thermal decomposition of peroxides; (3) rapid oxidation and thermal decomposition lead to
explosion hazard.
rsc.li/rsc-advances
12
cinnamaldehyde oxidation in acidic solution. However, cin-
namaldehyde can be oxidized without a catalyst when exposed
Introduction
Cinnamon is a special natural plant resource in Guangxi, China. to air at room temperature due to the unstable conjugated
Cinnamaldehyde is the main component in cinnamon oil, bonds and aldehyde group in its molecular structure. This
which is extracted from cinnamon by steam distillation. As an restricts the application of cinnamaldehyde. For instance, cin-
important spice and organic intermediate, it is widely used in namaldehyde is widely used as a food additive. Aer being
medicine and food due to its strong antibacterial, antiviral, and oxidized, cinnamaldehyde loses its sterilization and anti-
1–5
antiseptic properties. Cinnamaldehyde, as a kind of biomass corrosion functions, which can accelerate the quality deterio-
compound, is composed of a benzene ring, an unsaturated ration of food.
double bond, and an aldehyde group. Cinnamaldehyde shows
The oxidation of cinnamaldehyde is of concern at low
6
high oxidation reactivity in the presence of oxidants. Many temperatures. Due to the inevitable contact with air in the
studies on cinnamaldehyde oxidation have been reported, actual production, transportation, storage, and application,
13
which mainly focus on the reaction conditions of different cinnamaldehyde forms organic peroxides, which seriously
oxidants and catalysts. Cinnamaldehyde is particularly oxidized reduce its quality. Organic peroxides are hazardous materials,
7
14
by air in the presence of the N-heterocyclic carbene catalyst.
which are prone to thermal decomposition and explosion. The
The catalytic oxidation of cinnamaldehyde using oxygen, explosion in Brussels on March 22, 2016 was caused by tri-
NaClO, and H O as the oxidants with manganese porphyrin acetone triperoxide (TATP), which is a unstable peroxide and
2
2
15
and b-cyclodextrin as the catalysts showed the characteristics of prone to explosion. The structure of cinnamaldehyde has
free radical reaction. Yang studied the selective oxidation of a double bond and an aldehyde group, which is easy to oxidize.
cinnamaldehyde to benzaldehyde by H
with b-cyclodextrin In order to reduce or avoid the oxidation reaction of cinna-
8
–10
11
2 2
O
polymer as the catalyst in weak alkaline conditions. Iridium maldehyde, Schreck et al. reported the molecular encapsulation
trichloride-cerium sulfate has a strong catalytic activity for of cinnamaldehyde with cyclodextrin as an effective method to
improve the photostability and thermostability of
16
cinnamaldehyde.
School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004,
Guangxi, China. E-mail: xmliu1@gxu.edu.cn; Tel: +86 138 7713 6730
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The complexation of cinnamaldehyde with b-cyclodextrin magnetic stirrer, made in China, the heating medium is heat
17
can prevent cinnamaldehyde oxidation, which extends its conductive oil (glycerol). MCPVT could thus be used for evaluating
shelf life. Therefore, it is necessary to study the characteristics the oxidation reaction of cinnamaldehyde. Whether cinnamalde-
and hazard assessment of the oxidation process for cinna- hyde oxidation takes place aer the thermal decomposition of
maldehyde production, storage, and transportation.
cinnamaldehyde or whether it directly reacts with oxygen is worth
In the present work, the pressure and temperature behavior of studying. For understanding the characteristics of oxidation and
the cinnamaldehyde oxidation process was determined using the order of the thermal decomposition reaction, it is necessary to
MCPVT (mini closed pressure vessel test). The characteristics, study the pressure effects of cinnamaldehyde under oxygen and
kinetics, and thermal stability of cinnamaldehyde oxidation were nitrogen atmosphere. In order to avoid the inuence of metals on
investigated. It is very important to explore the evaluation cinnamaldehyde oxidation, 1 g cinnamaldehyde was put into a test
method and the possible explosion conditions of the oxidation tube (glass material) and a magnetic stirrer was put into the test
process of cinnamaldehyde. The study of the oxidation products tube, which was then set into the stainless steel reactor of MCPVT.
and reaction pathways is needed because it helps to understand The initial pressure was 0.57 MPa. The oxidation experiments were
the chemical properties and thermal stability of cinnamalde- carried out at isothermal conditions below the boiling point of
hyde. The purpose of this study is to supply the basic data so as to cinnamaldehyde (524 K) under stirring.
improve the safety of production, storage, and transportation of
The pressure and temperature changes were recorded by the
cinnamaldehyde, and to provide research methods for the recorder and the signal conditioner. Aer the reaction, the
organic synthesis and oxidation mechanism of cinnamaldehyde. products were cooled to room temperature and collected. Under
the same experimental conditions, nitrogen instead of oxygen
was used in a comparative experiment. In order to reduce the
experimental error, three parallel experiments were conducted
at each temperature.
Materials and experimental procedures
Materials
Cinnamaldehyde (99%, Shanghai Medical Technology Co.,
LTD, China), N
Co., LTD, China), KI (99.50%, Aladdin Industrial Corporation,
China), and Na (99.95%, Aladdin Industrial Corporation,
2 2
, O (99.99%, Nanning Zhong Yi Chuang Gas
Analysis of peroxide value
In order to detect the formation of cinnamaldehyde peroxides,
a proper amount of cinnamaldehyde was reacted with oxygen in
a three-neck ask under open conditions. The reaction was
conducted at different temperatures for 8 h with sampling aer
2 2 3
S O
China) were used in the experiments.
Cinnamaldehyde oxidation in MCPVT
18
every 2 h. The peroxide value were determined by iodimetry at
A mini closed pressure vessel test (MCPVT, vessel volume: 35 mL) different temperatures and times. About 0.1 g of the sample
was designed, as shown in Fig. 1. The main components of obtained from cinnamaldehyde oxidation was added to
MCPVT are: (a) a pressure sensor, made in Japan; (b) a tempera- aqueous potassium iodide. The peroxides were reduced by
ture sensor, made in Japan; (c) a recorder, made in Japan; (d) potassium iodide (eqn (I)). An equivalent amount of iodine was
a stainless steel high-pressure reactor, self-designed; (e) a gas liberated with starch and made the solution blue. Then, the
cylinder, made in China; (f) a heating and stirring system; the solution was titrated with standardized sodium thiosulfate
system is a DF-101S collector type constant temperature heating solution until the blue disappeared (eqn (II)). The experiment
Fig. 1 Experimental installation of the cinnamaldehyde oxidation reaction.
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was carried out three times in parallel to obtain the average
value. The results were quantied as milligrams per kilogram
ꢀ
1
(
mmol kg ) of peroxide.
KI + ROOH + H
+ 2Na
2
2
O / I
2
+ 2KOH + ROH
+ 2NaI
(I)
I
2
2
S
2
O
3
/ Na
2
S
4
O
6
(II)
Analysis of the oxidation products
GC-MS gas (chromatography-mass spectrometry) is an effective
means to analyse the oxidation products of cinnamaldehyde. It
is of signicance for the safe use of cinnamaldehyde, the
analysis of the oxidation mechanism, and thermal decomposi-
tion characteristics. The GC-MS instrument (GCMS-QP2010
Ultra, Shimadzu Corp.) for the analysis of the oxidation prod-
ucts contained an electron impact ionization detector (EID: 70
eV) and a DB-WAX fused silica capillary column (with an inner
diameter of 30 m ꢁ 0.25 mm and a lm thickness of 0.25 mm;
J&W Scientic Inc.). The following temperature program was
used: maintained 343.15 K for 3 min, rst increased to 450.15 K
Fig. 3 Pressure versus time for cinnamaldehyde, 1 – N
2
, 353 K; 2 – N
2
,
3
08 K; 3 – O , 308 K; 4 – O , 318 K; 5 – O , 328 K; 6 – O
2
2
2
2
, 338 K.
ꢀ1
ꢀ1
(
8 K min ) and then increased to 473.15 K (5 K min ), and
ꢀ1

nally increased to 513.15 K (20 K min ). The carrier gas was
high-purity helium gas, the column ow rate was 1.53
ꢀ1
mL min , the injection volume was 0.2 mL, the split ratio was
0.0 : 1, the inlet temperature was 523.15 K, and the interface
8
temperature was 503.15 K. The mass spectrometer used an EI
ion source with an electron energy of 0.8 kV and a mass spec-
trometer scanning range of 14–500 m/z in the full scan mode.
Results and discussion
Pressure behavior of the cinnamaldehyde oxidation process
Fig. 4 Temperature versus time for cinnamaldehyde, 1 – N
2
, 353 K; 2
, 308 K.
–
O
2
, 338 K; 3 – O , 328 K; 4 – O , 318 K; 5 – O , 308 K; 6 – N
2
2
2
2
Cinnamaldehyde is a compound with conjugated double
bonds. Its structure is shown in Fig. 2. According to the struc-
ture of cinnamaldehyde, one of its chemical properties is its
high reactivity. Therefore, it is necessary to investigate its
thermal stability. First, we investigated the thermal stability of
cinnamaldehyde. The MCPVT experiment was carried out under
nitrogen atmosphere. The temperature vs. time (T–t) and pres-
sure vs. time (P–t) plots of cinnamaldehyde are shown in Fig. 3
Cinnamaldehyde is a liquid (l) at room temperature and its
boiling point is 524 K. At the room temperature of ꢂ353 K and
MPa pressure, the vapor pressure of cinnamaldehyde is so low
that it can be ignored. Therefore, when using MCPVT to inves-
tigate reaction (III), it can be considered that the pressure
behavior of the oxidation process presents a change in the
oxygen (g) content. The cinnamaldehyde oxidation experiments
were conducted at 308 K, 318 K, 328 K, and 338 K in oxygen
atmosphere. The experimental results are shown in Fig. 3. Line
1
(line 1) and Fig. 4 (line 1), respectively. Then, we investigated the
stability of cinnamaldehyde under oxygen atmosphere. Cinna-
maldehyde oxidation is shown in reaction (III):
1
represents the nitrogen atmosphere, which was used as the
comparative experiment.
Cinnamaldehyde (l) + O
2
(g) / Products
(III)
Fig. 3 shows that the pressure remained constant aer the
temperature was increased to 353 K under nitrogen atmosphere,
which indicated that no gas consumption or thermal decompo-
sition reaction occurred. By contrast, the pressure dropped
rapidly aer heating to 308 K, 318 K, 328 K, and 338 K under
oxygen atmosphere. These results showed that oxidation took
place with the consumption of a large amount of oxygen. The
cinnamaldehyde oxidation process is a gas–liquid reaction. Cin-
namaldehyde absorbed oxygen to form non-gaseous products,
causing a decrease in the pressure in the MCPVT. Before 0.2 h,
cinnamaldehyde absorbed oxygen to form cinnamaldehyde
Fig. 2 The structure of cinnamaldehyde.
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peroxide and the pressure changed slowly. Then, the thermal
decomposition of cinnamaldehyde peroxides produced a great
quantity of free radicals, causing the fast radical oxidation of
cinnamaldehyde and the pressure showed almost a linear
decrease. Oxygen consumption increased with the increasing
temperature. Hence, the rst two stages of cinnamaldehyde
oxidation process can be described as follows: (1) cinnamalde-
hyde reacts with oxygen to form peroxides; (2) complex oxidation
reactions are caused by the thermal decomposition of peroxides.
In order to slow down the deterioration of cinnamaldehyde,
contact with oxygen should be prevented during the process of
production, storage, and transportation.
Kinetics of cinnamaldehyde oxidation
Fig. 5 Plots of (1/C
A
) versus time at different temperatures, 1 – 338 K;
Chemical kinetics is an important method to study the reac-
tivity, stability, and hazard of compounds. Cinnamaldehyde is
stable under nitrogen atmosphere but it is unstable under
oxygen atmosphere. Therefore, it is necessary to study the
oxidation kinetics of cinnamaldehyde at low temperature. It is
an attempt to determine the oxidation kinetics of cinnamalde-
hyde using MCPVT. It may include the kinetics of oxygen
absorption and the chemical reaction because it cannot
distinguish between the control steps in the mass transfer of the
oxygen absorption process and the oxidation reaction in
MCPVT. It is valuable for understanding the characteristics,
thermal stability, and risks of cinnamaldehyde oxidation.
It was assumed that cinnamaldehyde oxidation occurred as
follows:
2
– 328 K; 3 – 318 K; 4 – 308 K.
Then, eqn (3) can be obtained by the integral of eqn (2):
/(CA0 ꢀ x) ¼ kt + q
1
(3)
where q is a constant.
A
Fig. 5 showed the plots of (1/C ) versus t at 308 K, 318 K, 328
K, and 338 K, which indicated that ln k had a good linear rela-
tionship with t. The initial oxidation reaction of cinnamalde-
hyde is a second-order reaction.
Fig. 6 represents the ln k versus (1/T) curve. The tted linear
equation is given as:
2
Cinnamaldehyde (l) + O (g) / Products
t ¼ 0
C
C
A0
A0 ꢀ x
C
C
B0
0
x
ln k ¼ ꢀ2233.66 ꢁ (1/T) + 11.19
(4)
t ¼ t
B0 ꢀ x
2
Kinetic equation (eqn (1)):
The correlation coefficient (R ) is 0.995. The activation
energy (E ) of cinnamaldehyde oxidation obtained from the
(1) linear slope is 18.57 kJ mol . Table 1 showed that the reaction
a
a
b
ꢀ1
dx/dt ¼ k(CA0 ꢀ x) (C ꢀ x)
B0
rate increased with the increasing temperature at low temper-
ature. This could be attributed to the formation of reactive
where t is the time, CA0 is the initial concentration of cinna-
maldehyde, CB0 is the initial concentration of oxygen, x is the
molar number of the reaction products at t, k is the rate
constant of oxidation, and a and b are the reaction order of
cinnamaldehyde and oxygen, respectively. The kinetic calcula-
tions begun with the stabilized temperature, which was recor-
ded as 0 h.
If we make the following assumptions:
(
1) The oxidation kinetics is a second-order reaction (t ¼ 0.2–
1
.2 h).
(
(
2) Cinnamaldehyde vapor pressure is p ¼ 0.
3) Oxygen is an ideal gas, the remaining molar number of
oxygen can be calculated by the ideal gas equation: n ¼ (B ꢀ x)
t
¼
PV/RT, (P is the pressure, R is the gas constant, and T is the
temperature).
So, eqn (2) is simplied by eqn (1)
dx/dt ¼ k(CA0 ꢀ x)(CB0 ꢀ x)
(2)
For the experiment, nA0 ¼ nB0 ¼ 0.007955 mol. The initial
temperatures were 308 K, 318 K, 328 K, and 338 K, respectively. Fig. 6 Plot of ln k versus 1/T.
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Table 1 Rate constant of cinnamaldehyde oxidation at different
temperatures
ꢀ1
2
T (K)
k (h
)
R
3
3
3
3
08
18
28
38
99.23977
78.456743
63.75072
51.96077
0.9996
0.9982
0.9989
0.9985
19
radicals via the absorption of oxygen to form the peroxides.
Because of the low activation energy, cinnamaldehyde is easily
oxidized even at 308 K. As a traditional spice, the low temper-
ature oxidation of cinnamaldehyde leads to its deterioration
during storage, transportation, and usage.
Fig. 8 Peroxide value under normal pressure, 1 – 308 K; 2 – 318 K; 3 –
328 K; 4 – 338 K.
Peroxide value of cinnamaldehyde oxidation
The MCPVT results have shown that oxygen was absorbed by
cinnamaldehyde. It is worth noting that the initial oxidation decreased with the rising temperature from 318 K to 338 K,
reaction could form some products at low temperature. Does which demonstrated that the decomposition rate of peroxide
the initial oxidation reaction produce the hazardous substance was higher than its formation rate. Peroxide as the interme-
2
0
peroxide? Peroxide is a kind of hazardous compound, which is diate product is easy to decompose by heat, resulting in the
prone to thermal decomposition and is difficult to separate and deep oxidation of cinnamaldehyde. Organic peroxides ought
2
1
characterize due to its instability. In order to verify the forma- to be stored at low temperature as the self-decomposition
tion of the peroxide and investigate the effect of oxidation reaction of peroxide occurs when it exceeds the critical
temperature and time on the peroxide value, the peroxide temperature. The heat released from the reaction accelerates
concentrations were determined by iodimetry, which are its decomposition, where the decomposition temperature
expressed by the peroxide value. The effect of temperature on depends on the difficulty of its decomposition. The self-
the cinnamaldehyde peroxide value was evaluated at 308 K, 318 accelerated decomposition temperatures of stable peroxide
K, 328 K, and 338 K.
are in the range of 333–343 K, and the unstable ones are 293 K
The variation in the peroxide value with temperature under or even lower. So, the peroxide value rst increased then
high pressure is shown in Fig. 7. The peroxide value reached decreased with the rising temperature from 308 K to 338 K.
ꢀ
1
the maximum of 193.61 mmol kg at 318 K. This is a strong The cinnamaldehyde peroxides are more easily accumulated at
evidence for the formation of peroxide by cinnamaldehyde a lower temperature.
oxidation. The peroxide value increased with the increase in
The effect of reaction time on cinnamaldehyde peroxide
the temperature below 318 K, which indicated the accumula- under normal pressure was evaluated at 308 K, 318 K, 328 K,
tion of the peroxide. Following this, the peroxide value and 338 K. The variation in the peroxide value with reaction
time at different temperatures is shown in Fig. 8. The result
showed that the peroxide value increased constantly on pro-
longing the reaction time and reached the maximum value of
ꢀ
1
113.29 mmol kg
at 318 K within 8 h. Cinnamaldehyde
peroxide was continuously formed and accumulated as the
reaction time increased at low temperature. The longer the
reaction time, the higher was the cinnamaldehyde peroxide
value. Therefore, the optimal temperature for cinnamaldehyde
peroxide formation was 318 K.
When the temperature was higher than 318 K, cinnamalde-
hyde peroxide was greatly decomposed. The peroxide value
reached a maximum value when the reaction time was 8 h.
Cinnamaldehyde peroxide can affect the quality of cinna-
maldehyde and increase the thermal instability of cinna-
maldehyde. So, it is necessary to inhibit the formation of
cinnamaldehyde peroxide and study the possible thermal
hazards of the cinnamaldehyde oxidation process.
Fig. 7 Peroxide value under high pressure.
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Fig. 9 Explosion studies of cinnamaldehyde oxidation (a) temperature versus time (b) pressure versus time.
The hazards of the cinnamaldehyde oxidation process
acceleration decomposition temperature and the material
kinetic parameters for thermal decomposition. The device has
been widely applied in assessing the thermal stability of
Cinnamon oil (about 80% cinnamaldehyde) is obtained from
the bark and leaves of cinnamon by steam distillation. Also,
natural cinnamaldehyde is obtained by distillation of cinnamon
oil. In the process of production, storage, and transportation,
cinnamaldehyde may react with oxygen. Rapid oxidation reac-
tions are hazardous and can be converted into combustion or
explosion reactions. Therefore, it is important to study the
oxidation of cinnamaldehyde to avoid safety-related accidents.
There are many methods to evaluate the risk of chemical
reaction processes, such as Reaction Calorimetry (RC1) and
Advanced Reactive System Screening Tool (ARSST). Yang and
25
26,27
hazardous compounds. Liu
evaluated the thermal stability
16
of nine organic peroxides by MCPVT. Schreck used MCPVT to
study the explosive risk of mixtures of hydrogen peroxides and
alcohols. Therefore, the hazards of cinnamaldehyde oxidation
were studied using MCPVT in this section. The gas–liquid
oxidation was carried out with 1 g cinnamaldehyde under
0.72 MPa pressure of oxygen by MCPVT, which included
a recorder and a signal regulator to monitor the changes in the
temperature and pressure of the system. The curves of T–t
(temperature versus time) and P–t (pressure versus time) are
shown in Fig. 9(a) and (b), respectively.
According to Fig. 9, a violent thermal hazard was found in
cinnamaldehyde oxidation when the oxidation temperature was
raised to 345 K, accompanied by a sharp rise in the system
pressure. It indicated that the accumulation and rapid decom-
position of cinnamaldehyde peroxides released a large amount
of heat, which caused an increase in the system temperature
22
Leveneur et al. reported the risk assessment of hot air in the
hydrogenation of levulinic acid to g-pentalactone using ARSST.
It is interesting to study the risk of cinnamaldehyde oxidation
by MCPVT because it is a cheap and simple method.
The thermal decomposition violence assessment of organic
peroxide was studied by MCPVT (mini closed pressure vessel
23
24
test). MCPVT has shown its superiority in measuring the
Fig. 10 Explosion of cinnamaldehyde oxidation (a) dT/dt versus time (b) dP/dt versus time.
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0
Table 2 (dT/dt)max, (dP/dt)max, T and Tmax of cinnamaldehyde oxidation
Cinnamaldehyde
mass
ꢀ1
ꢀ1
T
0
(K)
T
max (K)
(dT/dt)max (K s
)
(dP/dt)max (MPa s
)
1
g
345
373
2.25
0.26
from 345 K to 373 K. The bond energy of O–O of the peroxides is proof material, as a safety device in MCPVT, can take
ꢀ
1
28
only 43 kcal mol , which is easy to break. When the system a maximum pressure of 10 MPa, which was broken inside the
temperature exceeds the self-decomposition temperature of the autoclave and turned black, and the glass container carrying the
peroxides, the thermal decomposition reaction of the peroxide reactants was crushed into pieces by the enormous explosive
will take place to produce small gaseous molecules and active power, as shown in Fig. 11. It is a strong evidence of the violent
free radicals.
explosion. The pressure sensor, which could withstand
Also, it will lead to the free radical oxidation reaction, which is a maximum pressure of 20 MPa, was broken. The actual
29
one of the main reasons for the explosion. The free radical maximum pressure could not be recorded due to the extremely
concentration and reaction chains increase exponentially when rapid explosion process. In addition, the explosion was aggra-
the large exothermic heat of the reaction system accumulates and vated by the presence of sufficient amount of oxygen. The
the total reaction rate is continuously accelerated. A large amount mixtures of cinnamaldehyde and the peroxide have high energy
of heat is released by the reaction cycle and leads to an uncon- content. Cinnamaldehyde oxidation is a dangerous process
30
trolled reaction process. Organic peroxide has strong decom- when the peroxide decomposition releases a large amount of
position characteristics, it produces a large amounts of heat and heat. Therefore, the thermal decomposition of cinnamaldehyde
31
gas aer decomposition, and can lead to runaway reactions.
peroxide and the mixtures would lead to the thermal hazards of
Peroxide decomposition generates a large number of reactive free cinnamaldehyde oxidation, which can further be converted to
32,33
radicals, which can initiate rapid oxidation reactions.
The an explosion. Also, the hazardous thermal process could be
cinnamaldehyde oxidation reaction is a remarkable exothermic described as the third stage of cinnamaldehyde oxidation.
reaction with a rapid rate at high temperature.
The rst derivatives of P–t and T–t are calculated to obtain
Products of cinnamaldehyde oxidation
the dP/dt–t and dT/dt–t curves, as shown in Fig. 10(a) and (b),
respectively. When the (dT/dt) or (dP/dt) is positively associated
with the reaction rate, it can be used for evaluating the hazards
The oxidation reaction products are indispensable for under-
standing the characteristics of the oxidation process, the reac-
tion pathway and mechanism, and the reaction hazards. The
initial oxidation products of cinnamaldehyde are important to
elucidate the mechanism and stability of the chemical reaction.
In order to study the initial oxidation products of cinna-
maldehyde oxidation and explore the pathways of oxidation, the
oxidation products of cinnamaldehyde at 308 K, 318 K, 328 K,
and 338 K for 6.5 h were analyzed by GC-MS. The results are
shown in Table 3.
26
of the oxidation process. Liu evaluated the safety of ethers,
abietic acid, rosin, and dimethoxymethane oxidation reactions
34–36
by this method.
The maximum rise rate temperature (dT/
dt)max and the maximum rise rate pressure (dP/dt)max are crit-
37
ical parameters to evaluate the stability of the reactant. As
shown in Table 2, dP/dt and dT/dt reached the maximum value
ꢀ1
ꢀ1
of 0.26 MPa s and 2.25 K s , respectively, when the system
temperature reached 373 K. This indicates that cinnamalde-
hyde oxidation can transform to an explosion easily.
According to the results of GC-MS, the similarity of each
component was retrieved from the mass spectrometry library.
The content of cinnamaldehyde was consistent with that of the
raw material in the contrast experiment and the other compo-
nents were not detected. It indicates that cinnamaldehyde
shows no reactivity under nitrogen atmosphere. On the
contrary, cinnamaldehyde is easy to oxidize under oxygen
atmosphere. The double bond outside the benzene ring was
broken to produce the epoxide of cinnamaldehyde (3-phenyl-
An explosion reaction occurred due to rapid oxidation when
the system temperature was increased to 373 K. The explosion-
6
oxirane-2-carbaldehyde) with the three-ring space congura-
tion and the epoxide continued to react with oxygen to generate
phenylacetaldehyde, benzaldehyde, benzoic acid, 2-hydrox-
yacetone, and other oxidation products. The oxygen consump-
tion and the cinnamaldehyde oxidation depth added to the
increase in the oxidation temperature. The degree of allyl
oxidation was deepened to form aldehydes, ketones, and alco-
hols, which continued to react with oxygen to produce deep
oxidation products such as benzoic acid and cinnamic acid. The
Fig. 11 The aftermath of cinnamaldehyde oxidation (a) round bottom
flask (b) broken flask after explosion.
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Table 3 Oxidation products of the cinnamaldehyde reaction
Reactive content
Molecular
formula
No.
Product
N
2
308 K
318 K
328 K
338 K
1
2
3
4
5
4
5
6
7
8
9
Water
Carbon dioxide
Acetaldehyde
Formic acid
Acetic acid
Benzaldehyde
Phenylacetaldehyde
Acetophenone
Phenylglyoxylic acid
Ethyl benzoate
H
CO
2
O
—
0.17
0.3
0.05
0.19
0.37
0.51
0.02
11.27
3.75
0.82
0.46
0.46
0.97
4.36
0.29
1.42
67.95
1.98
4.21
0.92
0.09
0.25
0.26
1.09
0.03
16.94
4.31
1.37
0.65
0.38
1.85
7.86
0.26
1.89
53.03
3.14
5.39
1.21
0.19
0.8
0.68
1.87
—
22
5.8
1.89
0.98
0.46
3.76
17.28
0.32
2.7
30.25
4.6
5.32
1.1
2
C
2
H
4
O
CH
2
O
2
0.3
C
C
C
C
C
C
C
C
C
C
C
C
C
2
7
8
8
8
9
7
9
H
H
H
H
H
H
H
H
4
6
8
8
8
O
O
O
O
O
O
O
O
2
0.03
7.91
2.89
0.26
0.31
0.21
0.12
1.27
0.18
0.94
78.76
1.32
3.58
1.3
2
10
2
Benzoic acid
6
2
10
11
12
13
14
15
16
3-Phenyloxirane-2-carbaldehyde (epoxide)
Ethyl phenylacetate
2-Hydroxyacetophenone
Cinnamaldehyde
Ethyl cinnamate
Cinnamic acid
8
2
10
H
12
O
2
2
8
H
8
O
O
12
O
2
9
H
8
99.57
11
9
H
H
8
O
2
Unknown component
hazard. For the safety of cinnamaldehyde production, storage,
and transportation, it is important to study the pathway of cin-
namaldehyde oxidation with cinnamaldehyde oxidation
products.
Pathways of cinnamaldehyde oxidation
The reaction pathway is very important to understand the
oxidation activity, stability, and hazards of cinnamaldehyde.
39
Friedman et al. reported the mechanism of the postulated
transformation of cinnamaldehyde to benzaldehyde and glyoxal.
The postulated mechanism involves the transformation of cin-
namaldehyde to the ring-opening peroxide. The peroxide bir-
adical then dimerizes to a cyclic dioxetane, which then undergoes
the indicated elimination to form the postulated products.
Fig. 12 Yield of the epoxide versus the reaction temperature.
cinnamaldehyde conversion increased with the rising temper-
ature. The reaction amount of cinnamaldehyde increased from
30% to 75% with an increase in the temperature rising from 308
K to 338 K under high pressure, which conrmed the process of
rapid oxygen consumption at 338 K. It indicates that both
temperature and oxygen concentration are important factors in
the cinnamaldehyde oxidation reaction.
The yield of the epoxide increased with rising temperature, as
38
shown in Fig. 12. Peroxide is a transitional molecule in the
formation of the epoxide. A higher temperature generates more
peroxide and intense peroxide decomposition triggers free
radical reactions to produce more epoxide. The results of the
peroxide value showed that more peroxide and epoxide were
produced under high pressure. The formation of epoxide also
reects the thermal decomposition of peroxide. The peroxide
decomposition reaction is a rapid exothermic process and
releases small gaseous substances, and it has a high thermal
Scheme 1 Pathway of cinnamaldehyde oxidation.
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40
Benzaldehyde, formaldehyde, or glyoxal were formed by ring-
opening.
Acknowledgements
From the analysis of peroxide and the oxidation products This work was supported by National Natural Science Foundation
and the mechanism reported in the literature, we proposed the of China (21776050), National Institute of Advanced Industrial
possible oxidation reaction pathway of cinnamaldehyde in Science and Technology Fellowship of Japan, Major Science and
MCPVT, which is shown in Scheme 1. Oxygen was absorbed by Technology Special Project in Guangxi (AA17204087-20).
cinnamaldehyde and the highly reactive double bond outside
the ring was destroyed to form the peroxide. A series of free
radical reactions was initiated by the peroxide to produce
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